1. Field of the Invention
The present invention relates to methods of forming stacked insulating films and of fabricating semiconductor devices, and more particularly, to stacked insulating films, methods for forming thereof, semiconductor devices using such stacked insulating film as an interlayer insulating film, and methods for fabricating such semiconductor devices.
2. Description of the Related Art
As a degree of integration of recent semiconductor integrated circuits advances, there is an increasing need for using a low-k film (a dielectric film having a low dielectric constant) as an insulating film aiming at faster operation speed and lower electric consumption of LSIs. An approach using an organic insulating film as the low-k film, in place of a silicon oxide film which has been a former standard of the interlayer insulating film, was reported, for example, by Hasegawa et al. in the Proceedings of 1997 Dry Process Symposium. The organic insulating film reported in the literature employed an organic polymer containing no silicon as a component element.
In the general procedures, such organic insulating film is processed using a process mask made of a silicon oxide film. Fukazawa et al. reported in the Proceedings of 1998 Dry Process Symposium an exemplary process based on dry etching using oxygen or nitrogen gas plasma. The silicon oxide film used as a process mask remains unremoved and serves as a part of a material composing a finished semiconductor device. The silicon oxide film can generally be processed with a fluorine-containing gas which hardly etches the organic insulating film. This ensures a status of so-called high process selectivity, which has been desirable in terms of designing semiconductor fabrication processes.
The silicon oxide film is generally formed by the plasma CVD (chemical vapor deposition) process, since the process temperature has to be suppressed at 400xc2x0 C. or below so as not to affect the wirings already formed. Typical conditions for the film formation employ monosilane (SiH4) with a flow rate of 100 sccm, nitrous oxide (N2O) with a flow rate of 2,000 sccm, and nitrogen (N2) with a flow rate of 1,000 sccm, all of which being introduced into a reaction chamber of a plasma CVD apparatus, and a microwave power of 350 W (2.45 GHz) and a substrate susceptor temperature of 400xc2x0 C.
However, a problem resides in that the interlayer insulating film made of an organic material may be highly combustible. Such organic insulating film can easily be processed with an oxygen-base gas, whereas it is very likely to get oxidation damage. Even in a process using nitrogen gas, the organic insulating film to be processed may easily get damage due to degassing components released from the neighboring oxide films. Although this may not result in combusting-out of the organic insulating film as in a process known as ashing, an oxidative decomposition reaction may proceed within the film, and volatile hydrocarbons (or oxygen-containing hydrocarbons) may be emitted. It is on such organic insulating film that the silicon oxide is stacked.
In the process of forming the silicon oxide film for covering wirings in LSIs, the plasma CVD process, is generally employed considering limitations on the film forming temperature and on the productivity. The reactive gas used in the plasma CVD process for forming the silicon oxide film, however, contains an oxidizing agent. For example, nitrous oxide is typically used as the oxidizing agent when silane gas is used as a silicon source, and oxygen is generally used for the case with tetraethoxysilane (TEOS).
After the silicon oxide film is formed to a certain thickness on the organic insulating film, the silicon oxide film per se becomes resistive enough to prevent the surface of the organic insulating film from direct attack by the oxygen gas plasma. Some fear of oxidative combustion reaction on the surface of the organic insulating film, however, still remains in the early stage of the film formation. While various approaches have been made to suppress the oxidative combustion reaction in the early stage of the film formation through controlling conditions of applying a microwave or a timing of the gas supply, they are still on the way to thorough suppression.
FIG. 7A shows an exemplary conventional aluminum wiring as combined with tungsten via plugs, in which a first wiring 111 is fabricated on a substrate 101, a first organic insulating film 112 is formed so as to cover the first wiring 111, and further thereon a first silicon oxide film 113 is formed. A first contact hole 114 is provided so as to penetrate the first silicon oxide film 113 and the first organic insulating film 112 and so as to reach the first wiring 111, and the first contact hole 114 is filled with a first plug 115 made of tungsten.
On the first silicon oxide film 113 formed is a second wiring 116, which is covered with a second organic insulating film 117, and further thereon a second silicon oxide film 118 is formed. A second contact hole 119 is provided so as to penetrate the second silicon oxide film 118 and the second organic insulating film 117 and so as to reach the second wiring 116, and the second contact hole 119 is filled with a second plug 120 made of tungsten.
FIG. 7B shows an exemplary copper damascene wiring, in which a first organic insulating film 211 is formed on a substrate 201, and further there on a first silicon oxide film 212 is formed. A first groove 213 is provided to the first silicon oxide film 212 and the first organic insulating film 211, and a first wiring 214 is formed so as to fill the first groove 213. On the first silicon oxide film 212, formed are a second organic insulating film 215 covering a first wiring 214; a second silicon oxide film 216; a third organic insulating film 217; and a third silicon oxide film 218 in this order.
A second groove 219 is provided to the third silicon oxide film 218 and the third organic insulating film 217, and a second wiring 220 is formed so as to fill the second groove 219. A first contact hole 221 is provided so as to penetrate the second silicon oxide film 216 and the second organic insulating film 215, and a first plug 222 is formed so as to fill the first contact hole 221 and so as to interconnect the second wiring 220 and the first wiring 214.
Further on the third silicon oxide film 218, a fourth organic insulating film 223 and a fourth silicon oxide film 224 are stacked in this order so as to cover the second wiring 220. A second contact hole 225 is provided so as to penetrate the fourth silicon oxide film 224 and the fourth organic insulating film 223 and so as to reach the second wiring 220, and a second plug 226 is formed so as to fill the second contact hole 225.
In both wiring configurations shown in FIGS. 7A and 7B, the individual organic insulating films may introduce damages in their interfacial area with the adjacent silicon oxide films when the silicon oxide films are directly formed on the organic insulating films .
Worse than all, even if the organic insulating films shown in the individual configurations in FIGS. 7A and 7B are made with, for example, polyaryl ether, which is expected to lower the overall dielectric constant of the insulating films due to its small dielectric constant of approx. 2.7, the effect of using such low-k film will partially be cancelled by using the silicon oxide films having a dielectric constant as high as 4.2. Hence, depending on the ratio of their film thicknesses, an effective dielectric constant of the stacked insulating film as contributed by the organic insulating films and the silicon oxide films will exceed 3.0. This may result in insufficient reduction in inter-wiring capacitance occurring between the neighboring wirings in the different layers or in the same layer, and thus may adversely affect performances of semiconductor devices such as signal transmission delay.
It is therefore an object of the present invention to provide stacked insulating films, methods for forming thereof, semiconductor devices using such stacked insulating film, and methods for fabricating such semiconductor devices.
A stacked insulating film of the present invention comprises an organic insulating film and a carbon-containing silicon oxide film formed thereon, in which the carbon-containing silicon oxide film has a carbon content of 8 atom % to 25 atom %.
As for the above stacked insulating film, the carbon-containing silicon oxide film formed on the organic insulating film serves as an etching mask when the organic insulating film is etched. The silicon oxide film with a carbon content of 8 atom % to 25 atom % has a dielectric constant of as low as approx. 2.0 to 3.0, which is smaller than that of silicon oxide films free from carbon or other impurities. Thus an effective dielectric constant of the stacked insulating film as contributed both by the organic insulating film and the carbon-containing silicon oxide film will be suppressed to 3.0 or below, thereby to provide the low-k insulating film.
A carbon content in the carbon-containing silicon oxide film exceeding 25 atom % may degrade the inorganic properties thereof and on the contrary enhance organic properties. In particular, a carbon content exceeding 30 atom % will almost ruin the inorganic properties of the carbon-containing silicon oxide film and yield organic properties instead. Such carbon-containing silicon oxide film will thus have an etching property similar to that of the organic insulating film lying thereunder, and can no longer play a role of an etching mask for the organic insulating film. On the contrary, a carbon content of the carbon-containing silicon oxide film less than 8 atom % may result in a high dielectric constant almost equivalent or similar to that of a carbon-free silicon oxide film, which is unsuccessful in fully obtaining effects of the carbon addition.
A method for forming a stacked insulating film of the present invention comprises a step for forming on a substrate an organic insulating film, and a step for forming by coating on the organic insulating film a carbon-containing silicon oxide film, in which the carbon-containing silicon oxide film is formed so as to attain a carbon content of 8 atom % to 25 atom %.
Such method for forming the stacked insulating film in which the carbon-containing silicon oxide film is formed by coating allows so-called low temperature film formation, and successfully avoids damages on the surface of the organic insulating film during the formation of the carbon-containing silicon oxide film since no plasma exposure nor sputtering action is involved. It also becomes proper to use the carbon-containing silicon oxide film as an etching mask when the organic insulating film is etched. Since the carbon-containing silicon oxide film is formed so as to attain a carbon content of 8 atom % to 25 atom %, the film will have a dielectric constant of as low as approx. 2.0 to 3.0, which is lower than that of a pure silicon oxide film. An effective dielectric constant of the stacked insulating film as contributed both by the organic insulating film and the carbon-containing silicon oxide film will be 3.0 or below, and thus the low-k stacked insulating film can be obtained. The reason why the carbon content is defined as above is similar to that described in the previous paragraph.
A semiconductor device of the present invention has an insulating film at least a part of which comprises a stacked insulating film, and such stacked insulating film has an organic insulating film and a carbon-containing silicon oxide film formed thereon, in which the carbon-containing silicon oxide film has a carbon content of 8 atom % to 25 atom %.
The semiconductor device has an insulating film at least a part of which comprises a stacked insulating film consisting of an organic insulating film and a carbon-containing silicon oxide film formed thereon, and the stacked insulating film has an effective dielectric constant lower than that of the conventional stacked insulating film consisting of an organic insulating film and a pure silicon oxide film (dielectric constant=4.2) free from impurities such as carbon. This is because silicon oxide film can reduce its dielectric constant by incorporating carbon, so that the above carbon-containing silicon oxide film can have a dielectric constant lower than that of silicon oxide film containing no impurity.
The carbon-containing silicon oxide film has a carbon content of 8 atom % to 25 atom % and, as a result, has a dielectric constant of approx. 2.0 to 3.0, which may vary depending on the carbon content. A dielectric constant of the organic insulating film is 3.0 or below in general. Thus the effective dielectric constant of the stacked insulating film as contributed by the organic insulating film and the carbon-containing silicon oxide film will be 3.0 or below, and thereby the low-k stacked insulating film can be composed. Using such stacked insulating film for isolating wirings in the different layers or within the same layer will successfully reduce the inter-wiring capacitance occurring between the neighboring wirings in separate layers or in the same layer, and will improve performances of semiconductor devices such as avoiding signal transmission delay. The reason why the carbon content is defined as above is similar to that described in the previous paragraph.
While the carbon-containing silicon oxide film, after being used as an etching mask for masking the organic insulating film, remains to be used as a part of the insulating film of semiconductor devices, the carbon-containing silicon oxide film will not adversely affect the lower dielectric strategy since the effective dielectric constant of the stacked insulating film will be regulated at 3.0 or below. This may increase the process margin in the forming process of the interlayer insulating film of semiconductor devices.
A method for fabricating a semiconductor device of the present invention comprises a step for forming on a substrate an organic insulating film, and a step for forming by coating on the organic insulating film a carbon-containing silicon oxide film, in which the carbon-containing silicon oxide film is formed by coating an organic spin-on-glass solution, and the carbon-containing silicon oxide film is formed so as to attain a carbon content of 8 atom % to 25 atom %.
Such method for fabricating a semiconductor device allows so-called low temperature film formation of the carbon-containing silicon oxide film since the film is formed by coating on the organic insulating film. It is also beneficial that the surface of the organic insulating film can exempt from process-related damages and thus ensure a good adhesiveness of the stacked insulating film, since no plasma irradiation nor sputtering action is exerted on the surface. For the case that the carbon-containing silicon oxide film is formed by coating an organic SOG (spin-on-glass) solution, the carbon content within the carbon-containing silicon oxide film can appropriately be adjustable according to structures and amount of carbon-containing groups in the organic SOG solution. For example, it is adjustable by substituting methyl groups with ethyl groups, or by altering the amount of alkyl groups. Thus the carbon-containing silicon oxide film can be formed with a desired carbon content.
Such carbon-containing silicon oxide film still retains inorganic properties, so that it can be useful as an etching mask when the organic insulating film is etched. Since the carbon-containing silicon oxide film is formed so as to attain a carbon content of 8 atom % to 25 atom %, the film will have a dielectric constant of as low as approx. 2.0 to 3.0, which is lower than that of a pure silicon oxide film. An effective dielectric constant of the stacked insulating film as contributed both by the organic insulating film and the carbon-containing silicon oxide film will be 3.0 or below, and thus the low-k stacked insulating film can be obtained. The reason why the carbon content is defined as above is similar to that described in the previous paragraph.